The solid biomass fuel having moisture content not exceeding 20% and 25mm in size is fed from the top of the Gasifier, while air and steam are fed from the bottom.The air and steam move upward against the downward movement of the biomass fuel, generating producer gas . This process of gas generation is the principle behind Updraft gasification.While the plant is under normal operation, following zones are believed to exist in the gasifier from bottom to top, as shown in the cross sectional view.

Ash Zone: A flow of heat takes place from hot ash to the air and water vapour entering the system and thus the hot process air enters the oxidation zone and the cooled ash leaves the system at the bottom to the water seal, generally maintained at about 600 mm wg.

Oxidation Zone : It is a narrow zone having the highest temperature(9500C–11000C) in the Gasifier due to the exothermic reaction:

C + O2 = CO2 + Heat

Primary Reduction Zone : The heat released by the oxidation zone below is cooled down by the endothermic reactions in this zone with the formation of CO, H2 and CO2 by the interaction of C present in the fuel with O2 and H2O present in the process air:

C + CO2 = 2CO ~ Heat

C + H2O = CO + H2 ~ Heat

C + 2H2O = CO2 + 2H2 ~ Heat

Secondary Reduction Zone : It reduces further the temperature of the gas coming out of the zone below due to endothermic reactions with the production of CO by the interaction of C with CO2 and simultaneous water gas reaction as follows:

C + CO2 = 2CO ~ Heat

CO + H2O = CO2 + H2 ~ Heat

Distillation zone : The heat of the upward moving gas from the secondary reduction zone helps the distillation of the volatile matters present in the fuel with the evolution of highly volatile hydrocarbons which combine with the gas to enrich it in its calorific value and the temperature of the gas goes down further .

The interior of the reactor is designed to provide adequate space and time for the above thermo-chemical reactions to enrich the gas and increase its calorific value.